Géométrie et structure électronique (méthode CNDO/2) du triala-1,3,5-tris(diméthyl)triaza-2,4,6-cyclohexahetrane [(CH

La validité relative des deux géométries récemment proposées, l’une (A) par spectrographie de rayons X et l’autre (B) par diffraction électronique, pour la molécule [(CH3)2NAlH2]3 est testée au moyen de la méthode CNDO/2. On montre ainsi que l’énergie totale obtenue avec la seconde est quelque 122 kcal/mole inférieure à celle à laquelle conduit la première. Il apparaît lors du calcul une interaction transannulaire [math] liante qui stabilise la conformation privilégiée et dont l’importance numérique (sensiblement moitié de l’énergie EN.Al associée à l’une des liaisons du cycle) souligne la nécessité d’utiliser une description topologique pour décrire la structure électronique de cycles renfermant plus d’un atome de la 3e ligne du tableau périodique

Molecular structure of tris(cyclopropylsilyl)amine as determined by gas electron diffraction and quantum-chemical calculations

Vishnevskiy YV, Abaev MA, Ivanov AA, Vilkov LV, Dakkouri M. Molecular structure of tris(cyclopropylsilyl)amine as determined by gas electron diffraction and quantum-chemical calculations. Journal of Molecular Structure. 2008;889(1-3):316-327.The molecular structure and conformation of tris(cyclopropylsilyl)amine (TCPSA) has been studied by means of gas-phase electron diffraction at 338 K and quantum-chemical calculations. A total of 12 relatively stable conformations of TCPSA molecule were considered. According to the experimental results and the DFT calculations the most stable conformer corresponds to a configuration (according to the Prelog–Klyne notation) of the type (−ac)(−ac)(+ac)-(−ac)(−ac)(+ac), where the first three parentheses describe the three different Si–N–Si–C torsional angles and the latter ones depict the rotation of the three cyclopropyl groups about the Cring–Si axes, respectively. The quantum-mechanical calculations were performed using various density functional (B3LYP, X3LYP and O3LYP) and perturbation MP2 methods in combination with double- and triple-ζ basis sets plus polarization and diffuse functions. The most important experimental geometrical parameters of TCPSA (ra Å, h1 degrees) are: (Si–N)av = 1.741(3) , (Si–C)av = 1.866(4), (C–C)av = 1.510(3), (C–C(Si))av = 1.535(3), (N–Si–C)av = 115.1(18)°. For the purpose of comparison and searching for reasons leading to the planarity of the Si3N moiety in trisilylated amines we carried out NBO analysis and optimized the geometries of numerous silylamines. Among these compounds was tris(allylsilyl)amine (TASA), which is isovalent and isoelectronic to TCPSA. Utilizing the structural results we obtained we could show that Si+Si+ electrostatic repulsive interaction is predominantly responsible for the planarity of the Si3N skeleton in TCPSA and in all other trisilylamines we considered. We also found that regardless the size and partial charges of the substituents the Si–N–Si bond angle in various disilylamines amounts to 130 ± 2°

Molecular Structure of 1,5-Diazabicyclo[3.1.0]hexane as Determined by Gas Electron Diffraction and Quantum-Chemical Calculations

Vishnevskiy YV, Vogt N, Vogt J, et al. Molecular Structure of 1,5-Diazabicyclo[3.1.0]hexane as Determined by Gas Electron Diffraction and Quantum-Chemical Calculations. The Journal of Physical Chemistry A. 2008;112(23):5243-5250.The equilibrium molecular structure and conformation of 1,5-diazabicyclo[3.1.0]hexane (DABH) has been studied by the gas-phase electron-diffraction method at 20 °C and quantum-chemical calculations. Three possible conformations of DABH were considered: boat, chair, and twist. According to the experimental and theoretical results, DABH exists exclusively as a boat conformation of Cs symmetry at the temperature of the experiment. The MP2 calculations predict the stable chair and twist conformations to be 3.8 and 49.5 kcal mol−1 above the boat form, respectively. The most important semi-experimental geometrical parameters of DABH (re, Å and ∠e, deg) are (N1−N5) = 1.506(13), (N1−C6) = 1.442(2), (N1−C2) = 1.469(4), (C2−C3) = 1.524(7), (C6−N1−C2) = 114.8(8), (N5−N1−C2) = 107.7(4), (N1−C2−C3) = 106.5(9), and (C2−C3−C4) = 104.0(10). The natural bond orbital (NBO) analysis has shown that the most important stabilization factor in the boat conformation is the n(N) → σ*(C−C) anomeric effect. The geometry calculations and NBO analysis have been performed also for the bicyclohexane molecule

Molecular structure, conformation, potential to internal rotation, and ideal gas thermodynamic properties of 3-fluoroanisole and 3,5-difluoroanisole as studied by gas-phase electron diffraction and quantum chemical calculations

Dorofeeva OV, Vishnevskiy YV, Rykov AN, et al. Molecular structure, conformation, potential to internal rotation, and ideal gas thermodynamic properties of 3-fluoroanisole and 3,5-difluoroanisole as studied by gas-phase electron diffraction and quantum chemical calculations. Journal of Molecular Structure. 2006;789(1-3):100-111

An alternative gas-phase electron diffraction and quantum chemical study of nitroethane

Shishkov IF, Sipachev VA, Dem'yanov PI, et al. An alternative gas-phase electron diffraction and quantum chemical study of nitroethane. Journal of Molecular Structure. 2010;978(1-3):41-47.The results of alternative gas-phase electron diffraction (GED) study of nitroethane are compared with those reported in the previous study by Tarasov et al. [1]. A fairly large reliability factor (R = 6.1%) obtained in that work, even though the intensity data over the range s > 27 Å-1 were excluded from the analysis, was one of the reasons for undertaking this investigation. Our structural analysis was performed with electron diffraction patterns used by Tarasov et al. In this work, various quantum chemical models were used as initial approximations, and two sets of GED data equally well fitted to theory were obtained. For the experimental intensity curve in the range of s = 3.8-32.6 Å-1, the disagreement between the theoretical and experimental data reached R = 4.3%. The natural bond orbital (NBO) analysis and the quantum theory of atoms in molecules (QTAIM) were used to reveal the reasons of a low barrier to rotation of nitro group

Molecular structure and conformation of nitrobenzene reinvestigated by combined analysis of gas-phase electron diffraction, rotational constants, and theoretical calculations

Dorofeeva OV, Vishnevskiy YV, Vogt N, et al. Molecular structure and conformation of nitrobenzene reinvestigated by combined analysis of gas-phase electron diffraction, rotational constants, and theoretical calculations. Structural Chemistry. 2007;18(6):739-753